Microlithographic device, microlithographic assist features, system for forming contacts and other structures, and method of determining mask patterns

ABSTRACT

A method of formulating and fabricating a mask pattern and resulting mask for forming isolated or closely spaced contact holes in an integrated circuit. The mask has a transparent mask substrate and patterned regions of attenuating phase shift material and opaque, partially transmissive or transparent material arranged to reduce the effect of side lobes and improve depth of focus. The rims, frames and outrigger patterns for the attenuating phase shift material and opaque, partially transmissive or transparent material are determined according to calculations performed on a processor with imaging software for various optical conditions and exposed feature criteria.

BACKGROUND OF THE INVENTION

[0001] 2. Field of the Invention

[0002] The present invention relates to masks (or reticles) and otherdevices for use in microlithographic techniques, especially for use informing contact holes in semiconductor products with improved depth offocus. The invention may be used to form isolated contact holes, arraysof contact holes and other structures. The present invention alsorelates to a method of determining a desired three-tone pattern for amicrolithographic mask. The method, which may be performed on one ormore programmed microprocessors, may involve the selection of one set ofdimension data out of a plurality of dimension data sets, where thedimension data sets correspond to different mask patterns, and theselected set is the one that provides the greatest depth of focus.

[0003] 2. Description of Related Art

[0004] A semiconductor device can be fabricated by photolithography, inwhich light is transmitted through a patterned mask (or reticle). Thepattern on the mask is exposed on a layer of photoresist to form thedesired feature or features in the semiconductor device. Examples ofsuch features are isolated contact holes and contact holes formed inclosely packed arrays. In certain circumstances, it is desirable to makecontact holes with very small critical dimensions. The “criticaldimension” is typically the diameter of the hole in the plane of thesurface of the semiconductor device. Some devices require contact holeswith critical dimensions that are less than the wavelength of the lightthat is used to expose the photoresist. A dimension that is less thanthe wavelength of the exposing light is referred to as a“sub-resolution” dimension.

[0005] A number of binary and phase-shifting masks have been proposed inthe prior art. Such masks are shown, for example, in U.S. Pat. No.6,114,071 (Chen et al.), U.S. Pat. No. 6,077,633 (Lin et al.), U.S. Pat.No. 6,022,644 (Lin et al.) and U.S. Pat. No. 5,707,765 (Chen). There isstill a need in the art, however, for a three-tone mask (or othermulti-tone mask) that can form holes and other structures with smallcritical dimensions and minimum side-lobing and with improved depth offocus. Depth of focus is especially important in connection with theformation of small structures in non-flat surfaces. Where the wafer isnot flat, it may be necessary to image a pattern at different distancesfrom the lithography system with essentially the same fidelity. Inaddition, it may be necessary to allow for wafer positioning in thesystem, wafer curvature, focal plane curvature, etc.

[0006] Moreover, there is a need in the art for an economical method ofmaking three-tone masks (or other multi-tone masks) for use in theformation of small critical dimension features with minimal side-lobingand large depth of focus.

SUMMARY OF THE INVENTION

[0007] The present invention relates to a microlithographic mask forforming a sub-resolution feature in photoresist with improved depth offocus. As noted above, the term “sub-resolution” means that the criticaldimension of the feature formed in the photoresist is less than thewavelength of the exposing light. According to one aspect of theinvention, the mask has a three-tone structure, with a layer oftransparent material, a layer of attenuating phase-shifting materialoverlying the transparent material, and a layer of light-obstructingmaterial (i.e., opaque material and/or partially transmissive material)overlying the phase-shifting material. In a preferred embodiment of theinvention, the layer of attenuating phase-shifting material is locatedbetween the transparent material and the light-obstructing material. Thepresent invention should not be limited, however, to the specificfeatures of the preferred embodiments shown and described in detailherein.

[0008] According to another aspect of the invention, opaque material andthe attenuating phase-shifting material are patterned to form a squaretransparent opening, one or more partially transmissive assist features,which may include a rectangular frame, and an opaque frame and/orbackground. The opaque frame may be located at the edge of the opening(interposed between a partially transmissive assist feature and thetransparent opening). Alternatively, opaque squares, triangles or otherpolygons can be placed at the corners (or inside) of a partiallytransmissive frame to control the exposure pattern of the light that istransmitted through the partially transmissive frame.

[0009] The transparent material may be quartz or another suitablematerial. The partially transmissive material causes a phase shift(e.g., 180° or an odd multiple thereof) relative to the lighttransmitted through the transparent material. The partially transmissivematerial also attenuates the phase-shifted light relative to thenon-phase-shifted light. The transmissivity of the partiallytransmissive material relative to the transparent material may be in therange, for example, of from about 6% to 100%, more preferably from about8% to about 24%. The partially transmissive material may be, forexample, MoSi. The opaque material may be a metal such as chrome, andother suitable materials may be employed as desired.

[0010] The present invention may be used to form a variety ofmicrolithographic features. The invention is especially well suited,however, for forming a contact hole that has a large aspect ratio ofdepth to width. The invention is also well suited to forming otherstructures where a large depth of focus is desirable, such asmicrolithographic features on substantially non-flat surfaces. Accordingto one aspect of the invention, improved depth of focus is achieved byproviding sub-resolution assist features that are patterned in theopaque material and/or the partially transmissive material.

[0011] The present invention also relates to masks for forming regularand asymmetric arrays of features, such as arrays of high aspect ratiocontact holes. According to one aspect of the invention, elongatedassist bars (of partially transmissive material and/or opaque material)are employed to interact with an array of transparent openings.According to another aspect of the invention, phase-shifting assistfeatures are nested within transparent bars.

[0012] The present invention also relates to a method of formingelliptical holes and other structures with small critical dimensions andimproved depth of focus. The holes may be isolated structures or theymay be formed in a dense array.

[0013] The present invention also relates to a method of making amulti-tone microlithographic mask. The method, which may be performed atleast in part on a digital microprocessor, includes the steps of: (1)providing sets of dimension data representative of multiple maskpatterns; (2) for each set of dimension data, calculating featuredimension data as a function of optical conditions; and (3) for adesired optical condition, identifying the sets of dimension data thatcorrelate to feature dimension data within desired limits. If desired,the method may also include the step of (4) selecting the one identifiedset of dimension data that achieves the smallest change in criticaldimension between a zero defocus condition and a maximum considereddefocus condition.

[0014] In a preferred embodiment of the invention, steps (1) and (2) areperformed using a computer programmed with PERL/solid-c imagingsoftware. Steps (3) and (4) may be performed using Visual BASIC/Excelsoftware. As noted above, however, the present invention should not belimited to the specific features of the preferred embodiments.

[0015] The dimension data can include the widths of transparent openingsand the corresponding dimensions of the opaque and partiallytransmissive assist features. There may be one set of such dimensiondata for each pattern under consideration. The limits considered in step(3) may include the critical dimension for the exposed feature, theallowable (or desirable) ellipticity, the absence of sidelobes,log-slope, etc. These limits operate to exclude patterns that do notform acceptable features at the desired operating conditions. Once adesired pattern is determined, the pattern is formed in layers ofdeposited partially transmissive and opaque materials to form thefinished mask. As noted above, the two upper layers of the mask may bedeposited on a layer of transparent quartz.

[0016] These and other advantages and features of the invention will bemore readily understood from the following detailed description of theinvention which is provided in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a plan view of a mask constructed in accordance with apreferred embodiment of the invention;

[0018]FIG. 2 is a cross-sectional view of the mask of FIG. 1, takenalong the line 2-2;

[0019]FIG. 3 is a plan view of another mask constructed in accordancewith the invention;

[0020]FIG. 4 is a cross-sectional view of the mask of FIG. 3, takenalong the line 4-4;

[0021]FIG. 5 is a plan view of another mask constructed in accordancewith the invention;

[0022]FIG. 6 is a cross-sectional view of the mask of FIG. 5, takenalong the line 6-6;

[0023]FIG. 7 is a top view of another mask constructed in accordancewith the invention;

[0024]FIG. 8 is a cross-sectional view of the mask of FIG. 7, takenalong the line 8-8;

[0025]FIG. 9 is a plan view of another mask constructed in accordancewith the invention;

[0026]FIG. 10 is a cross-sectional view of the mask of FIG. 9, takenalong the line 10-10;

[0027]FIG. 11 is a plan view of another mask constructed in accordancewith the invention;

[0028]FIG. 12 is a cross-sectional view of the mask of FIG. 11, takenalong the line 12-12;

[0029]FIG. 13 is a plan view of another mask constructed in accordancewith the invention;

[0030]FIG. 14 is a cross-sectional view of the mask of FIG. 13, takenalong the line 14-14;

[0031]FIG. 15 is a cross-sectional view of the mask of FIG. 13, takenalong the line 15-15;

[0032]FIG. 16 is a plan view of another mask constructed in accordancewith the invention;

[0033]FIG. 17 is a cross-sectional view of the mask of FIG. 16, takenalong the line 17-17;

[0034]FIG. 18 is a cross-sectional view of the mask of FIG. 16, takenalong the line 18-18;

[0035]FIG. 19 is a cross-sectional view of the mask of FIG. 16, takenalong the line 19-19;

[0036]FIG. 20 is a plan view of another mask constructed in accordancewith the invention;

[0037]FIG. 21 is a cross-sectional view of the mask of FIG. 20, takenalong the line 21-21;

[0038]FIG. 22 is a cross-sectional view of the mask of FIG. 20, takenalong the line 22-22;

[0039]FIG. 23 is a plan view of another mask constructed in accordancewith the invention;

[0040]FIG. 24 is a cross-sectional view of the mask of FIG. 23, takenalong the line 24-24;

[0041]FIG. 25 is a cross-sectional view of the mask of FIG. 23, takenalong the line 25-25;

[0042]FIG. 26 is a plan view of another mask constructed in accordancewith the invention;

[0043]FIG. 27 is a cross-sectional view of the mask of FIG. 26, takenalong the line 27-27;

[0044]FIG. 28 is a cross-sectional view of the mask of FIG. 26, takenalong the line 28-28;

[0045]FIG. 29 is a plan view of another mask constructed in accordancewith the invention;

[0046]FIG. 30 is a cross-sectional view of the mask of FIG. 29, takenalong the line 30-30;

[0047]FIG. 31 is a cross-sectional view of the mask of FIG. 29, takenalong the line 31-31;

[0048]FIG. 32 is a plan view of another mask constructed in accordancewith the invention;

[0049]FIG. 33 is a cross-sectional view of the mask of FIG. 32, takenalong the line 33-33;

[0050]FIG. 34 is a cross-sectional view of the mask of FIG. 32, takenalong the line 34-34;

[0051]FIG. 35 is a plan view of another mask constructed in accordancewith the invention;

[0052]FIG. 36 is a cross-sectional view of the mask of FIG. 35, takenalong the line 36-36;

[0053]FIG. 37 is a cross-sectional view of the mask of FIG. 35, takenalong the line 37-37;

[0054]FIG. 38 is a plan view of another mask constructed in accordancewith the invention;

[0055]FIG. 39 is a cross-sectional view of the mask of FIG. 38, takenalong the line 39-39;

[0056]FIG. 40 is a cross-sectional view of the mask of FIG. 38, takenalong the line 40-40;

[0057]FIG. 41 is a plan view of another mask constructed in accordancewith the invention;

[0058]FIG. 42 is a cross-sectional view of the mask of FIG. 41, takenalong the line 42-42;

[0059]FIG. 43 is a cross-sectional view of the mask of FIG. 41, takenalong the line 43-43;

[0060]FIG. 44 is a plan view of another mask constructed in accordancewith the invention;

[0061]FIG. 45 is a cross-sectional view of the mask of FIG. 44, takenalong the line 45-45;

[0062]FIG. 46 is a cross-sectional view of the mask of FIG. 44, takenalong the line 46-46; and

[0063]FIG. 47 is a flow chart for a method of designing amicrolithographic mask in accordance with a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0064] Referring now to the drawings, where like reference numeralsdesignate like elements, there is shown in FIG. 1 a microlithographicmask 30 for forming an isolated contact hole (not shown) in photoresist(not shown). The mask 30 is formed of a transparent substrate 32 (FIG.2), an attenuating phase shift layer 34, and an opaque layer 36. Theattenuating phase shift layer 34 and the opaque layer 36 are patternedto define a square transparent opening 38, a rectangular opaque frame 40(FIG. 1), a partially transmissive outrigger frame 42, and an opaquebackground 44. In the illustrated embodiment, the inner edges of theopaque frame 40 define transparent opening 38.

[0065] In operation, incident light 46 (FIG. 2) is transmitted throughthe opening 38 and the outrigger frame 42. The incident light 46 may begenerated by a suitable source (not shown) located above the mask 30.The incident light 46 is prevented from passing through the opaquematerial 40, 44. The light 46, 48 that is transmitted through theoutrigger frame 42 is phase-shifted (by 180° or an odd multiple thereof)relative to the light 46, 50 that is transmitted through the transparentopening 38. In addition, the outrigger frame 42 attenuates thephase-shifted light 48 relative to the non-phase-shifted light 50. Theattenuated phase-shifted light 48 interacts with the non-phase-shiftedlight 50 to form the contact hole in the photoresist.

[0066] In the illustrated embodiment, the opaque frame 40 causes thepartially transmissive outrigger frame 42 to be effectively spaced apartfrom the edges 52 of the transparent opening 38. That is, the opaqueframe 40 is interposed between the transparent opening 38 and thepartially transmissive frame 42. By blocking the incident light 46between the transparent opening 38 and the outrigger frame 42, thecontact hole can be formed in the shape of a cylinder (with minimal sidelobing) with improved depth of focus. In a preferred embodiment, thecylindrical contact hole may be formed with a depth of focus of about0.8 microns (μm) or greater. The depth of focus determines the length ofthe cylindrical hole that can be formed in the photoresist withoutunacceptable side-lobing. The depth of focus also characterizes theability of the mask 30 to be used to form sub-resolution features innon-flat photoresist surfaces.

[0067] In the illustrated embodiment, the substrate 32 is formed ofquartz, the attenuating phase shift layer 34 is formed of MoSi, and theopaque layer 36 is formed of chrome. The transmissivity of theattenuating phase shift layer 34 may be about 18% when the wavelength ofthe incident light is about 248 nanometers (nm). The transmissivity ofthe transparent quartz layer 32 may be essentially 100%. Other suitablematerials may be employed in the mask 30, and additional layers may beprovided, if desired. Further, the width 54 of the transparent opening38 is about 0.14 μm. The diameter of the hole (not shown) formed by themask 30 is about 0.12 μm (less than the wavelength of the incident light46) in the exposed photoresist. The width 56 of the opaque frame 40 isabout 0.125 μm, and the width 58 of the outrigger frame 42 is about0.115 μm. The incident light 46 is propagated with a numerical aperture(NA) of about 0.63 and a sigma (a) of about 0.35. The thickness of thethree layers 36, 34, 32 may be 700 to 1000 Angstroms, 800 to 1200Angstroms, and one-quarter inch, respectively. The present inventionshould not be limited, however, to the specific materials, dimensionsand instrumentalities of the preferred embodiments shown and describedin detail herein. As mentioned below, the scope of the invention shouldbe defined by the appended claims.

[0068] Referring now to FIGS. 3 and 4, there is shown a mask 70 forforming an isolated cylindrical contact hole (not shown) in photoresist(not shown). The mask 70 includes a transparent substrate 32, anattenuating phase shift layer 34, and an opaque layer 36. Theattenuating phase shift layer 34 and the opaque layer 36 are patternedto define a square transparent opening 72, rectangular, nested opaqueframes 74, 76, rectangular, nested partially transmissive outriggerframes 78, 80, and an opaque background 82.

[0069] In operation, incident light 46 is transmitted through theopening 72 and the outrigger frames 78, 80. The incident light 46 isprevented from passing through the opaque material 74, 76, 82. The light46, 48 that is transmitted through the outrigger frames 78, 80 isphase-shifted (by 180° or an odd multiple thereof) relative to the light46, 50 that is transmitted through the transparent opening 72. Inaddition, the phase-shifting layer 34 attenuates the phase-shifted light48 relative to the non-phase-shifted light 50. The attenuatedphase-shifted light 48 interacts with the non-attenuated,non-phase-shifted light 50 to form the cylindrical contact hole in theunderlying, exposed photoresist (not shown).

[0070] In the illustrated embodiment, the opaque frames 74, 76 separatethe partially transmissive outrigger frames 78, 80 from the edges 84 ofthe transparent opening 72 (and from each other). By blocking theincident light 46 between the transparent opening 72 and the outriggerframes 78, 80, the mask 70 of FIGS. 3 and 4 is able to form acylindrically-shaped hole with a critical dimension (CD) of 0.12 μm andwith a depth of focus of 0.8 μm or more. As in the embodiment of FIGS. 1and 2, the attenuating phase shift layer 34 is formed of MoSi, and theopaque layer 36 is formed of chrome. The relative transmissivity of thephase shift layer 34 may be about 18%. The width 86 of the transparentopening 72 is about 0.16 μm. The width 88 of each opaque frame 74, 76 isabout 0.085 μm, and the width 90 of each outrigger frame 78, 80 is about0.11 μm. The mask 70 would be suitable for operation under the sameoptical and photoresist conditions described above in connection withFIGS. 1 and 2 (i.e., where NA is about 0.63, and a is about 0.35).

[0071] The structures shown in FIGS. 1-4 are frame-based structures. Ineach illustrated embodiment there is opaque material abutting thecontact. Polygons of attenuated phase shift material, particularlyscattering bars, lay around the contact. The claimed invention shouldnot be limited, however, to the embodiments shown and described indetail herein.

[0072] The structures shown in FIGS. 5-8 are rim-based structures. Ineach embodiment there is attenuated phase shift material abutting thecontact. Polygons, either opaque or transparent, or of anothertransmission, lay immersed inside the attenuated material or at eitherborder, defining the form and size of the rim.

[0073] Referring now to FIGS. 5 and 6, there is shown a mask 100 forforming an isolated cylindrical contact hole with improved depth offocus. The mask 100 includes a transparent substrate 32, an attenuatingphase shift layer 34, and an opaque layer 36. The attenuating phaseshift layer 34 and the opaque layer 36 are patterned to define atransparent opening 102, a partially transmissive rim 104, opaque cornersquares 106, and an opaque background 108. In operation, incident light46 is transmitted through the opening 102 and the rim 104. The incidentlight 46 is prevented from passing through the opaque material 106, 108.The light 46, 48 that is transmitted through the rim 104 is phaseshifted (by 180° or an odd multiple thereof) and attenuated (to about18%) relative to the light 46, 50 that is transmitted through theopening 102.

[0074] As in the embodiments of FIGS. 1-4, the attenuated, phase-shiftedlight 48 interacts with the non-attenuated, non-phase-shifted light 50to form the contact hole with the desired geometry. The four opaquecorner squares 106 cover the partially transmissive layer 34 and therebycontribute to the formation of a cylindrically-shaped contact holewithout sidelobes. In the illustrated embodiment, the corner squares 106separate the partially transmissive rim 104 into four short bars. Byblocking the incident light 46 at points between the transparent opening102 and the opaque background 108, the contact hole can be formed withminimal side-lobing and with improved depth of focus.

[0075] In the illustrated embodiment, the width 110 of the transparentopening 102 is about 0.21 μm, and the CD of the hole (not shown) formedby the mask 100 is about 0.12 μm. The width 112 of the partiallytransmissive rim 104 is about 0.24 μm, and the width 114 of the squarecorners 106 is about 0.19 μm. The mask 100 may be operated under thesame optical and photoresist conditions as those described above inconnection with FIGS. 1 and 2.

[0076] The mask 130 shown in FIGS. 7 and 8 is similar to the mask 100shown in FIGS. 5 and 6 except that the opaque corners 132 in the FIGS. 7and 8 embodiment are triangular to provide improved side-lobing control.

[0077] The structures shown in FIGS. 9-12, like those shown in FIGS.1-4, are frame-based structures. Referring now to FIGS. 9 and 10, thereis shown yet another mask 150 for forming an isolated cylindricalcontact hole with improved depth of focus. The mask 150 has a squaretransparent opening 152, a partially transmissive short bar frame 154,opaque corner squares 156 at the ends of the short bars of the frame154, and an opaque background 158. In operation, incident light 46 istransmitted through the opening 152 and the frame 154. The light 46 isprevented from passing through the opaque material 156, 158, 160. Thelight 46, 48 that is transmitted through the short bar frame 154 isphase-shifted (by 180° or an odd multiple thereof) and attenuated (toabout 18%) relative to the light 46, 50 that is transmitted through theopening 152. The attenuated phase-shifted light 48 interacts with thenon-phase-shifted light 50 to expose the photoresist such that theformed hole has the desired geometry.

[0078] The opaque frame 160, like the frame 40 of the FIGS. 1 and 2embodiment, optically separates the partially transmissive frame 154from the transparent opening 152, to achieve improved depth of focus.The opaque corner squares 156 operate like the squares 106 of FIGS. 5and 6 to spatially and selectively control the transmission of lightthrough the frame 154, to thereby make it easier to ensure that thecontact hole has the desired cylindrical geometry, depth of focus andeliminate sidelobes.

[0079] In the illustrated embodiment, the width 162 of the transparentopening 152 is about 0.15 μm. The CD of the hole (not shown) formed bythe mask 150 is about 0.12 μm. The width 164 of the partiallytransmissive frame 154 is about 0.15 μm, and the width 166 of theoverlapping square corners 156 is likewise about 0.15 μm. The width 168of the opaque frame 160 (i.e., the separation of the partiallytransmissive frame 154 from the transparent opening 152 in orthogonaldirections) is about 0.10 μm. The mask 150 may be operated under thesame optical and photoresist conditions as those described above inconnection with FIGS. 1 and 2.

[0080] The mask 180 shown in FIGS. 11 and 12 has nested opaque frames182, 184 and nested, short bar, partially transmissive frames 186, 188.In the illustrated embodiment, the width (or separation distance) 190 ofeach opaque frame 182, 184 is about 0.12 μm, and the width 192 of eachpartially transmissive short bar 186, 188 is about 0.175 μm. The width194 of each overlapping opaque corner 196 is likewise about 0.175 μm.The width 198 of the square opening 199 is about 0.14 μm. Thus, thesmall dimensions of the features patterned on the mask 180 are all lessthan the wavelength of the incident light 46.

[0081] Further, the layered structure 32, 34, 36 may be patterned toform a mask for forming an array of closely-spaced contact holes (orother features in the photoresist). The array may be regularly orasymmetrically configured. The contact holes may be cylindrical orelliptical, as discussed in more detail below.

[0082]FIGS. 13-25 show frame-based structures for forming arrays ofcontact holes. Referring now to FIGS. 13-15, there is shown a mask 200for forming a regular array of contact holes, where each contact holehas a cylindrical CD of 0.12 μm, for example. The mask 200 is formed ofa transparent substrate 32, an attenuating phase shift layer 34, and anopaque layer 36. The layers 32, 34, 36 may be formed of the samematerials as described above in connection with FIGS. 1 and 2. Theattenuating phase shift layer 34 and the opaque layer 36 are patternedto define square transparent openings 202, rectangular opaque frames 204surrounding the openings 202, and partially transmissive/phase shiftingoutrigger bars 206. The partially transmissive outrigger bars 206 areconnected to each other to define a regular array of outrigger framesaround each opaque frame 204.

[0083] In operation, the mask 200 operates generally like the structureof FIGS. 1 and 2, except in a closely packed array. Incident light 46(FIG. 14) is transmitted through the openings 202 and the outriggerframes 206, but is prevented from passing through the opaque material204. The light that is transmitted through the outrigger frames 206 isphase-shifted (by 180° or an odd multiple thereof) and attenuated (toabout 18%) relative to the light 46, 50 that is transmitted through theopenings 202. The attenuated phase-shifted light 48 (FIG. 14) and thenon-phase-shifted light 50 interact with each other to form the desireddensely packed array of contact holes in the photoresist. The opaqueframes 204 operate to separate the partially transmissive material 34,206 from the square openings 202 to thereby provide the mask 200 withimproved depth of focus (in the range of from about 0.4 to 0.8 μm orgreater).

[0084] In the illustrated embodiment, the width 208 of the transparentopenings 202 is about 0.15 μm. All of the small dimensions of the assistfeatures 204, 206 are sub-resolution. The separation width 209 of therectangular frames 204 is about 0.15 μm. The widths 210, 212 of theoutrigger bars of the partially transmissive frame 206 are differentfrom each other, and may be about 0.12 μm and about 0.14 μm in therespective orthogonal directions. As in the embodiments of FIGS. 1-12,the incident light 46 is propagated with a NA of about 0.63 and anon-axis illumination σ of about 0.35. As noted, however, the presentinvention should not be limited to the specific materials, dimensionsand instrumentalities of the preferred embodiments shown and describedin detail herein. In all of the embodiments described herein, the NA maybe as high as 0.70 or more, and the C value may be in the range of fromabout 0.3 to 0.85, for example. The invention may be operated withon-axis and off-axis illumination systems.

[0085] The mask 240 shown in FIGS. 16-19 is the same as the one shown inFIGS. 13-15, except that the partially transmissive outrigger frame forthe mask 240 is formed of short bars 242, 244 that are separated attheir ends. Opaque squares 246 are formed between the ends of the shortbars 242, 244 to facilitate the design of the mask 240. The opaquesquares 246 spatially and selectively limit the amount of phase-shiftedlight 48 (FIG. 17) that is exposed on the photoresist (not shown) tothereby control the formation of the contact holes with the desiredgeometries and with improved depth of focus. In the embodiment of FIGS.16-19, the width 248 of each square portion 244 of the short bar frameis about 0.14 μm, and the width 250 of each elongated bar portion 242 ofthe short bar frame is about 0.12 μm. An advantage of the presentinvention is that the widths 248, 250 of the various cooperating assistfeatures 244, 242 may be different from each other.

[0086]FIGS. 20-22 show a mask 260 for forming an asymmetric array ofcylindrical contact holes (not shown). The contact holes are alignedwith transparent openings 262 patterned through the opaque layer 36 andthe partially transmissive layer 34. The opaque layer 36 is furtherpatterned to expose bar-shaped regions 264 of the partially transmissivematerial 34. The opaque material 36 that remains between the openings262 and the partially transmissive bars 264 operate like opaque framesof the type shown in FIGS. 1 and 2. The dimensions of the asymmetricmask 260 may be as follows: The width 266 of the square openings 262 maybe about 0.19 μm. The separation widths 268, 270 of the opaquerectangular frames 36 may be about 0.21 and 0.19 μm, respectively. Thewidth 272 of the elongated partially transmissive bars 264 may be about0.15 μm.

[0087] In an alternative embodiment of the invention (not shown), thepatterned layers of the mask 260 may be patterned in an inverse fashionsuch that the partially transmissive material 34 is contiguous with thetransparent openings 262, and bounded by longitudinal bars of opaquematerial 36. In this alternative embodiment of the invention, theincident light 46 is transmitted (50) through the openings 262 withoutany phase-shifting or attenuation. The incident light 46 is blockedentirely by longitudinal, parallel bars of opaque material 36, and therest of the incident light 46 (48) is attenuated and phase-shifted byassist frames that surround the transparent openings 262.

[0088]FIGS. 23-25 show a mask 300 for forming an array of ellipticalholes. The mask 300 of the FIGS. 23-25 embodiment is like the mask 260shown in FIGS. 20-22 except that the transparent openings 302 arerectangular and not square. In the FIGS. 23-25 embodiment, the width 304and length 306 of the openings 302 may be about 0.15 and 0.23 μm,respectively. The separation widths 308, 310 of the opaque rectangularframes 36 may be different from each other, and the length and width ofthe partially transmissive bars may be about 0.14 μm and 0.70 μm,respectively.

[0089]FIGS. 26-34 show rim-based structures for forming arrays ofcontact holes. Referring now to FIGS. 26-28, there is shown amicrolithographic mask 500 for forming a regular array of contact holes(not shown) in photoresist (not shown). The mask 500 is formed of atransparent substrate 32 (FIG. 27), an attenuating phase shift layer 34,and an opaque layer 36. The attenuating phase shift layer 34 and theopaque layer 36 are patterned to define rectangular transparent openings502, partially transmissive rims 504, and an opaque background 506.

[0090] In the structure shown in FIG. 26, the width and length of theopaque bars 36 between contacts 502 limit the size of the rims 504. Inan alternative embodiment of the invention, the bars surrounding therims 504 may be non-opaque, but of another transmission. Whether thebars are opaque, light-obstructing, or transparent, they can stillcontribute to the function of defining the image.

[0091] In operation, incident light 46 (FIG. 27) is transmitted throughthe openings 502 and the rims 504. The incident light 46 may begenerated by a suitable source (not shown) located above the mask 500.The incident light 46 is prevented from passing through the opaquematerial 506. The light 46, 48 that is transmitted through the rims 504is phase-shifted (by 180° or an odd multiple thereof) relative to thelight 46, 50 that is transmitted through the transparent openings 502.In addition, the rims 504 attenuate the phase-shifted light 48 relativeto the non-phase-shifted light 50. The attenuated phase-shifted light 48interacts with the non-phase-shifted light 50 to form the desiredcontact holes.

[0092] In the illustrated embodiment, the partially transmissive rims504 cooperate with the square openings 502 to form the contact holeswith minimal side lobing and improved depth of focus. In a preferredembodiment, the cylindrical contact hole may be formed with a depth offocus of about 0.8 μm or greater. As in the embodiments discussed above,the depth of focus determines the length of the cylindrical hole thatcan be formed in the photoresist without unacceptable side-lobing. Thedepth of focus also characterizes the ability of the mask 500 to be usedto form sub-resolution features in non-flat photoresist surfaces.

[0093] In the illustrated embodiment, the substrate 32 is formed ofquartz, the attenuating phase shift layer 34 is formed of MoSi, and theopaque layer 36 is formed of chrome. The transmissivity of theattenuating phase shift layer 34 may be about 18% when the wavelength ofthe incident light is about 248 nm. The transmissivity of thetransparent quartz layer 32 may be essentially 100%. Other suitablematerials may be employed in the mask 500, and additional layers may beprovided, if desired.

[0094] Further, the orthogonal dimensions 508, 510 of each transparentopening 502 are about 0.22 μm and 0.20 μm, respectively. The diameter ofeach hole (not shown) formed by the mask 500 is about 0.12 μm (less thanthe wavelength of the incident light 46) in the exposed photoresist. Thewidths 512, 514 of the opaque bars 506 are about 0.10 μm and 0.06 μm,respectively. The incident light 46 is propagated with a numericalaperture (NA) of about 0.63 and a sigma (σ) of about 0.35. The thicknessof the three layers 36, 34, 32 may be 700 to 1000 Angstroms, 800 to 1200Angstroms, and one-quarter inch, respectively. The present inventionshould not be limited, however, to the specific materials, dimensionsand instrumentalities of the preferred embodiments shown and describedin detail herein.

[0095]FIGS. 29-31 show another microlithographic mask 550 for forming aregular array of contact holes in photoresist (not shown). The mask 550is formed of a transparent substrate 32 (FIG. 30), an attenuating phaseshift layer 34, and an opaque layer 36. The three layers 32, 34, 36 maybe formed of the same materials and with the same thicknesses asdescribed above in connection with the mask 500 of FIGS. 26-28.Alternatively, other suitable materials may be employed in the mask 550,and additional layers may be provided, if desired.

[0096] In the FIGS. 29-31 embodiment, the attenuating phase shift layer34 and the opaque layer 36 are patterned to define square transparentopenings 552, partially transmissive rims 554, and an opaque backgroundformed of short bars 556. In operation, incident light 46 is transmittedthrough the openings 552 and the rims 554. The incident light 46 isprevented from passing through the opaque bars 556. The light 46, 48that is transmitted through the rims 554 is phase-shifted (by 180° or anodd multiple thereof) relative to the light 46, 50 that is transmittedthrough the transparent openings 552. In addition, the rims 554attenuate the phase-shifted light 48 relative to the non-phase-shiftedlight 50. The attenuated phase-shifted light 48 interacts with thenon-phase-shifted light 50 to form the desired array of contact holeswith minimal side lobing and improved depth of focus.

[0097] The width 558 of each transparent opening 552 is about 0.21 μm.The diameter of each hole (not shown) formed by the mask 550 is about0.12 μm in the exposed photoresist. The width 560 of each short opaquebar 556 is about 0.10 μm. The incident light 46 may be propagated with anumerical aperture (NA) of about 0.63 and a sigma (σ) of about 0.35inch. The present invention should not be limited, however, to thespecific materials and dimensions shown and described in detail herein.The scope of the invention should be determined according to theappended claims.

[0098]FIG. 32 shows another microlithographic mask 600 for forming aregular array of contact holes in photoresist. As shown in FIGS. 33 and34, the illustrated mask 600 is formed of a transparent substrate 32, anattenuating phase shift layer 34, and an opaque layer 36. The threelayers 32, 34, 36 may be formed of the same materials and with the samethicknesses as described above in connection with the mask 500 of FIGS.26-28. The attenuating phase shift layer 34 and the opaque layer 36 arepatterned to define rectangular transparent openings 602, partiallytransmissive, asymmetric rims 604, and an opaque background formed oflong bars 606. The parallel bars 606 are staggered with respect to theopenings 602. In operation, incident light 46 is transmitted through theopenings 602 and the asymmetric rims 604.

[0099] The incident light 46 is prevented from passing through theopaque bars 606. The light 46, 48 that is transmitted through the rims604 is phase-shifted (by 180° or an odd multiple thereof) relative tothe light 46, 50 that is transmitted through the transparent openings602. In addition, the rims 604 attenuate the phase-shifted light 48relative to the non-phase-shifted light 50. The attenuated phase-shiftedlight 48 interacts with the non-phase-shifted light 50 to form thedesired array of closely packed contact holes with minimal side lobingand improved depth of focus.

[0100] In the illustrated embodiment, the orthogonal dimensions 608, 610of each transparent opening 602 are about 0.19 μm and 0.21 μm,respectively. The diameter of each hole (not shown) formed by the mask500 is about 0.12 μm in the exposed photoresist, where the propagationcharacteristics of the incident light are as follows: NA=about 0.63; andσ=about 0.35. To define the desired asymmetric rims 604, each longopaque bar may have a width 610 of about 0.15 μm and a length 612 ofabout 0.72 μm. The dimensions may be calculated by a suitable programmedcomputer as discussed in more detail below.

[0101]FIGS. 35-46 show additional rim-based structures for formingisolated contacts. Thus, FIG. 35 shows a microlithographic mask 650 forforming an isolated contact hole in photoresist. As shown in FIGS. 36and 37, the mask 650 includes a transparent substrate 32, an attenuatingphase shift layer 34, and an opaque layer 36. The three layers 32, 34,36 may be formed of the same materials and with the same thicknesses asdescribed above in connection with the mask 500 of FIGS. 26-28. Theattenuating phase shift layer 34 and the opaque layer 36 are patternedto a square transparent opening 652 enclosed within concentric partiallytransmissive frames 654, 656, 658. The partially transmissive assistfeatures 654-658 are defined by a double set of long, orthogonal, opaquebars 660, 662, and an outer opaque background 664.

[0102] In operation, incident light 46 is transmitted through theisolated opening 652 and the concentric rims 654-658. The incident light46 is prevented from passing through the opaque frames 660, 662. Thelight 46, 48 that is transmitted through the rims 654-658 isphase-shifted (by 180° or an odd multiple thereof) relative to the light46, 50 that is transmitted through the transparent opening 652. The rims654-658 attenuate the phase-shifted light 48 relative to thenon-attenuated light 50. The light 48, 50 interacts to form the desiredcontact hole with minimal side lobing and improved depth of focus.

[0103] In the illustrated embodiment, the width 664 of the opening 652is about 0.20 μm to form a 0.12 μm diameter hole in the exposedphotoresist, where the propagation characteristics of the incident light46 are as follows: NA=about 0.63; and σ=about 0.35. The width 668 ofeach long bar frame 660, 662 may be about 0.07 μm. If desired, theseparation distances 670 between the opaque assist features may be about0.1 μm.

[0104]FIGS. 38-40 show a microlithographic mask 700 with a rim thatincludes double transparent long bars. The mask 700 may be used to forman isolated contact hole in photoresist. Like the masks discussed above,the mask 700 of FIGS. 38-40 is formed of a transparent substrate 32, anattenuating phase shift layer 34, and an opaque layer 36. As shown inFIG. 38, the attenuating phase shift layer 34 is patterned to form asquare transparent opening 702 (width 704=about 0.22 μm) and concentrictransparent frames 706, 708 (width 710=about 0.08 μm). The opening 702is surrounded by the transparent frames 706, 708. The partiallytransmissive material 34 on opposite sides of the rims 706, 708 formsphase-shifting assist features 712, 714, 716. The assist features712-716 (separation distance 718=about 0.2 μm) may be concentrically,alternatingly nested with the transparent frames 706, 708. The outermostassist feature 712 may be surrounded by the opaque background 36.

[0105] In operation, incident light 46 (NA=about 0.63; σ=about 0.35) istransmitted through the isolated opening 702, the transparent frames706, 708 and the rim shaped assist features 712-716. The light 46, 48that is transmitted through the concentric rims 712-716 is phase-shifted(by 180° or an odd multiple thereof) and attenuated relative to thelight 46, 50 that is transmitted through the transparent opening 702 andthe frames 706, 708. Thus, the transmitted light 48, 50 interacts toform the desired contact hole (not shown; diameter=about 0.12 μm) withminimal side lobing and improved depth of focus.

[0106]FIGS. 41-43 show a microlithographic mask 750, with a rim thatincludes double opaque short bars, for producing an isolated contacthole in photoresist (not shown; diameter=about 0.12 μm). Like the masksdiscussed above, the mask 750 is formed of a transparent substrate 32,an attenuating phase shift layer 34, and an opaque layer 36. Throughoutthis specification, as noted above, like reference numerals designatelike elements. Referring now to FIGS. 42 and 43, the attenuating phaseshift layer 34 and the opaque layer 36 may be patterned to form a squaretransparent opening 752 (or “contact”; width 754=about 0.2 μm) andconcentric frames formed of short bars 756, 758 (width 760=about 0.08μm) whose ends do not overlap each other.

[0107] The transparent opening 752 is surrounded by the orthogonallyarranged short bars 756, 758. The opaque frames 756, 758 define phaseshifting assist features 764, 766, 768 in the partially transmissivelayer 34. The separation distance 770 between the opaque frames 756, 758may be about 0.1 μm, although changes may be made to the illustratedembodiments without departing from the scope of the present invention.The assist features 764, 766, 768 may be concentrically, alternatinglynested with the opaque frames 756, 758. The outermost assist feature 764may be surrounded by an opaque background 780. The opaque background 780is formed by a non-patterned region of the opaque layer 36.

[0108] In operation, incident light 46 (NA=about 0.63; σ=about 0.35) istransmitted through the isolated opening 752 and the assist features764, 766, 768. The light 46, 48 that is transmitted through the assistfeatures 764, 766, 768 is phase-shifted (by 180° or an odd multiplethereof) and attenuated relative to the light 46, 50 that is transmittedthrough the transparent opening 752. Thus, the transmitted light 48, 50interacts to form the desired contact hole with minimal side lobing andimproved depth of focus.

[0109]FIG. 44 shows a microlithographic mask 800, with a rim thatincludes double transparent short bars, for producing an isolatedcontact hole in photoresist (not shown; diameter=about 0.12 μm). Themask 800 may be constructed generally like the mask 750 discussed abovein connection with FIGS. 41-43 except that the concentric frames aretransparent (i.e., patterned through the partially transmissive material34) rather than partially transmissive (i.e., patterned only through theopaque layer 36). Referring now to FIGS. 45 and 46, the mask 800 mayhave a square transparent opening 802 (contact width 804=about 0.21 μm)and concentric frames formed of transparent short bars 806, 808 (width810=about 0.08 μm) whose ends do not overlap each other.

[0110] The transparent opening 802 is surrounded by concentric assistfeatures 812, 814 separated by and nested within the transparent bars806, 808. The illustrated separation distance 816 may be, for example,about 0.22 μm. The opaque background 818 is formed by a non-patternedregion of the device 800. In operation, incident light 46 (NA=about0.63; σ=about 0.35) is transmitted through the isolated opening 802, theclear outrigger frames 806, 808, and the assist features 812, 814. Theattenuated and phase shifted light 46, 48 interacts with the non-phaseshifted light 46, 50 to form the desired contact hole with minimal sidelobing and improved depth of focus.

[0111] In connection with FIGS. 1-46, all of the dimensions provided forthe illustrated masks are at 1×, i.e., at the final dimensions on thewafer or photoresist within which the contact hole(s) is(are) formed.The masks may be fabricated at a larger scale, for example, at 4× or 5×,such that when the geometry is imaged onto the wafer, the image isshrunk by the scale factor. In addition, satisfactory results may beobtained even through the dimensions are not exactly as specified in thedetailed description. For example, each dimension may be in a tolerancerange of 10% plus or minus the value specified herein. The inventionshould not be limited to the specified dimensions and ranges, however,except to the extent such values are recited in the claims.

[0112] Further, please note that, although the structures shown in FIGS.35-46 have two scattering bars per side, the invention may be employedwith any number of bars per side, including one. The claimed inventionshould not be limited to the preferred embodiments shown and describedin detail herein.

[0113] Referring now to FIG. 47, there is shown a flow chart for amethod of making and/or designing a multi-tone microlithographic mask inaccordance with one aspect of the present invention. The method may bearranged to identify or select the one mask pattern, out of numerousmask patterns of the types shown in FIGS. 1-46, that provides thegreatest depth of focus under given optical and feature conditions. Thegiven conditions may relate to the optical and operational parameters ofthe microlithography system that will be used, the type of photoresistwithin which the contact holes or other features will be formed, and thedesign criteria, limits or tolerances on the geometry of the features tobe formed in the photoresist. The latter design limits may include thedesired critical dimension (CD), lack of side-lobing, etc. In apreferred embodiment of the invention, the design criteria may alsoinclude the image log-slope. The term “image log-slope,” which is a wellknown term in the art, basically refers to the slope of the diffractionpattern. Higher slope provides an image with sharper edges, andtherefore improved contrast.

[0114] In the illustrated method, sets of dimension data are input (Step400) into a programmed microprocessor or the like. The sets of dimensiondata are representative of the planar dimensions of various maskpatterns. Each set of dimension data may include the width of thetransparent opening(s), the width or separation distance of any opaqueframe(s), the width, separation distance or other planar dimensions ofany partially transmissive rims, the widths of corner squares (opaque orpartially transmissive), the relevant dimensions of other assistfeatures, etc. The dimensions that make up the dimension data may bevaried within predetermined ranges. The process of varying the dimensiondata and inputting the sets of dimension data into the system (Step 400)may be automated.

[0115] In a subsequent step (Step 402), calculations are carried out foreach set of dimension data. The calculations provide feature data, foreach set of dimension data, as a function of optical conditions. Thefeature data may be, for example, representative of the CD, ellipticity,side-lobing, etc. of the features that would be produced by a maskhaving the dimensions of the respective dimension data set. The featuredata is calculated as a function of optical conditions such as intensityrange, NA, a, etc. In addition, the feature data is calculated for zero,intermediate and maximum defocus conditions. At the conclusion of Step402, there are multiple sets of feature data for each set of dimensiondata.

[0116] Subsequently, in Step 404, for a desired optical condition, thesets of dimension data that result in feature data within acceptablelimits (design criteria) are identified. The number of sets of dimensiondata output from Step 404 is less than the number of sets input in Step400. For example, for each optical condition (e.g., intensity or dose)considered in a range of optical condition values, the programinvestigates the calculated CD values as a function of dimension data,and discriminates according to the following design criteria: (1) CDs atzero, intermediate and maximum considered defocus conditions to bewithin tolerance of desired value; (2) ellipticity ((CDx−CDy)/(CDx+CDy))to be within desired limits at zero defocus as well as on defocus.According to a preferred embodiment of the invention, the desired CD ata zero defocus condition may be about 0.12 μm, with a tolerance of about±3%. The CD tolerance at an intermediate defocus condition (e.g., 0.40μm) may be ±15%. The CD tolerance at a maximum considered defocuscondition (e.g., ±0.6 um) may be, for example, ±30%.

[0117] Then, in Step 406, the one set of dimension data, from among thesets selected in Step 404, that achieves the smallest change in criticaldimension (ACD) between a zero defocus condition and a maximumconsidered defocus condition is obtained by a sorting process. Note thatthe “intermediate defocus” is considered to provide a process latitudewindow within defocus values; however, to investigate depth of focus, aCD value is obtained at the maximum considered defocus condition.

[0118] Steps 400 and 402 may be performed on a microprocessor programmedwith suitable imaging software, such as, for example, Solid C, AerialImage version 5.5.11. The computer may be programmed in PERL script todefine, write, and execute macros to analyze various surface dimensionslike those mentioned above in connection with FIGS. 1-46.

[0119] Thus, the invention provides a method of forming a mask and theresulting structure to form contact holes in photoresist onsemiconductor wafers. In an exemplary embodiment, a mask is formed witha transparent material mask substrate, attenuating phase shift materialformed on the substrate, and opaque material regions formed on theattenuating phase shift material. The attenuating phase shift materialand opaque (or partially transmissive) material are patterned to form atransparent hole. The dimensions of the patterns are determined usingiterative methods and imaging software. An automated method is used toselect the most desirable pattern for given conditions and designcriteria. Certain dimensions, such as the size of the transparentopening and the size and spacing of the attenuating phase shift materialand opaque material may be set as critical limits to reduce side lobesfor a given illumination condition.

[0120] After a desired pattern is selected, then a mask, with thedesired sub-resolution dimensions, is formed in the three-layer material32, 34, 36 using electron beam lithography, ion milling, etc. Processesfor forming the desired pattern in the three layer material 32-36 aredescribed in U.S. Pat. No. 5,582,939 (Pierrat), for example. The entiredisclosure of U.S. Pat. No. 5,582,939 is incorporated herein byreference.

[0121] The method described above may also be used to optimize the sizeof lines (as opposed to contacts) as well as the position and size ofscattering bars. The method is applicable to the formation of linesand/or slots as well as for contacts.

[0122] Having thus described in detail certain exemplary embodiments ofthe invention, it is to be understood that the invention defined by theappended claims is not to be limited by particular details set forth inthe above description as many apparent variations thereof are possiblewithout departing from the spirit or scope of the invention.Accordingly, the above description and accompanying drawings are onlyillustrative of exemplary embodiments which can achieve the features andadvantages of the invention. It is not intended that the invention belimited to the embodiments shown and described in detail herein. Theinvention is only limited by the scope of the following claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A microlithographic mask for forming asub-resolution feature in photoresist with an acceptable processlatitude, said mask comprising: a layer of transparent material; a layerof light-obstructing material; and a layer of attenuating phase shiftingmaterial located between said layer of transparent material and saidlayer of light-obstructing material; and wherein said layer oflight-obstructing material and said layer of attenuating phase shiftingmaterial are patterned to form a transparent hole, a partiallytransmissive assist feature, and a light-obstructing frame locatedbetween said transparent hole and said partially transmissive assistfeature.
 2. The mask of claim 1, wherein said transparent hole is arectangle.
 3. The mask of claim 2, wherein said light-obstructing frameincludes an opaque frame, and wherein said opaque frame surrounds saidtransparent hole.
 4. The mask of claim 3, wherein said partiallytransmissive feature includes a partially transmissive frame surroundingsaid opaque frame.
 5. The mask of claim 4, wherein said layer oflight-obstructing material includes a layer of opaque material, andwherein said layer of opaque material includes an opaque backgroundsurrounding said partially transmissive frame.
 6. The mask of claim 1,wherein said layer of transparent material includes quartz.
 7. The maskof claim 6, wherein said layer of attenuating phase shifting materialincludes a material selected from the group consisting of MoSi, chromiumfluoride, silicon nitride, titanium nitride, tantalum silicide andzirconium silicon oxide.
 8. The mask of claim 7, wherein saidattenuating phase shifting material is deposited on said quartz.
 9. Themask of claim 7, wherein the transmissivity of said layer of attenuatingphase shifting material relative to said layer of transparent materialis in the range of from about 6% to 100%.
 10. The mask of claim 7,wherein said layer of light-obstructing material includes chrome. 11.The mask of claim 1, further comprising a partially transmissive frame,said layer of phase-shifting material being located between saidtransparent hole and partially transmissive frame. 12-22. (canceled) 23.A microlithographic mask, comprising: transparent material; andpatterned opaque material and phase shifting material, said patternedmaterials defining an opening, an opaque frame surrounding said opening,sub-resolution bars surrounding said frame, and opaque corners locatedbetween sub-resolution bars.
 24. The mask of claim 23, wherein saidtransparent material includes quartz.
 25. The mask of claim 24, whereinsaid phase shifting material is partially transmissive relative to saidtransparent material.
 26. The mask of claim 25, wherein said opaquematerial includes metal deposited on said phase shifting material. 27.(canceled)
 28. A mask for forming an array of sub-resolution features,said mask comprising: a layer of transparent material; a layer oflight-obstructing material; and a layer of attenuating phase shiftingmaterial located between said layer of transparent material and saidlayer of light-obstructing material; and wherein said layer oflight-obstructing material and said layer of attenuating phase shiftingmaterial are patterned to form transparent holes and light-obstructingframes surrounding said transparent holes.
 29. The mask of claim 28,further comprising partially transmissive features surrounding saidfight-obstructing frames.
 30. The mask of claim 29, further comprisingopaque corners on said partially transmissive features.
 31. A multi-tonemask for forming sub-resolution features, said mask comprising: a firstlayer of attenuating phase shifting material, said layer definingopenings corresponding to said sub-resolution features; and a secondlayer of material for preventing incident light from propagating throughsaid first layer, said second layer including frames surrounding saidopenings, and wherein said second layer defines bar-shaped partiallytransmissive assist features.
 32. The mask of claim 31, whereinphase-shifted light transmitted through one of said bar-shaped partiallytransmissive assist features operatively interacts with lighttransmitted through said openings.
 33. A mask for forming an ellipticalfeature in photoresist, said mask comprising: a layer of transparentmaterial; a layer of opaque material; and a layer of attenuating phaseshifting material located between said layer of transparent material andsaid layer of opaque material; and wherein said layer of opaque materialand said layer of attenuating phase shifting material are patterned toform transparent holes and opaque frames surrounding said transparentholes.
 34. The mask of claim 33, further comprising an array ofrectangular openings, and attenuating phase shifting bars locatedbetween said rectangular openings.
 35. The mask of claim 34, furthercomprising a patterned layer of opaque material for defining saidattenuating phase shifting bars.
 36. The mask of claim 35, furthercomprising a transparent substrate for supporting said attenuating phaseshifting bars and said patterned layer of opaque material.
 37. A methodof making a multi-tone microlithographic mask, said method comprising:providing sets of dimension data representative of mask patterns; foreach set of dimension data, calculating feature dimension data as afunction of optical conditions; and for a desired optical condition,identifying the sets of dimension data that have feature dimension datawithin desired limits.
 38. The method of claim 37, further comprisingthe step of selecting the one set of dimension data that achieves thesmallest change in critical dimension between a zero defocus conditionand a maximum considered defocus condition.
 39. The method of claim 38,wherein said dimension data includes the widths of transparent openingsin said patterns. 40-46. (canceled)
 47. The method of claim 37, whereinsaid feature dimension data define opaque corner structures.
 48. Themethod of claim 37, wherein said feature dimension data define partiallytransmissive corner structures.